Figure 1.

Neurophysiological short- (<7 days) and long (>7 days) term- adaptation to microgravity and ground-based model. Upper left panel: the mass of the otoconia increases following short-term exposure to microgravity (Vinnikov Ia et al., 1980; Ross et al., 1985; Lychakov et al., 1989). This increase in mass is assumed to be maintained as long as there is no change in the gravitational environment. The opposite phenomenon occurs in hypergravity (decrease in mass; Krasnov, 1991; Pedrozo and Wiederhold, 1994; Anken et al., 1998; reviewed in Cohen et al., 2005). To date, however, the time course is unknown since testing was only done after long-term centrifugation (>24 days). Middle left panel: After 7–9 days in microgravity, type II vestibular hair cells increase in size and number (Ross, 1993, 1994, 2000). Then after 2 weeks in microgravity, the number of type I cells increases as well (Ross and Tomko, 1998). In hypergravity, only type II hair cells show a significant decrease in number (e.g., following 14–30 days of centrifugation; Lychakov et al., 1989; Ross, 1993). Bottom left panel: Immediately after entering microgravity, studies across species have reported increases in vestibular afferent baseline activity and sensitivity (Gualtierotti and Alltucker, 1967; Gualtierotti, 1977; Boyle et al., 2001). However, when considered alone, studies in NHPs have been inconclusive as some report increases and other report decreases in sensitivity (Correia et al., 1992; Cohen et al., 2005). After 5 days in microgravity, vestibular afferent responses return to ground levels (Bracchi et al., 1975; Boyle et al., 2001). On earth, 1 month after complete unilateral vestibular lesion, afferent responses in the intact nerve remain comparable to control levels (Sadeghi et al., 2007, 2010, 2011, 2012). Bottom right panel: Across animal models, the sensitivities of vestibular nuclei neurons initially increase in microgravity and then return to baseline levels after a week (Pompeiano et al., 2002; Cohen et al., 2005). On earth, following labyrinthectomy, vestibular nuclei neurons that normally only respond to vestibular input before lesion, show the emergence of responses to extravestibular inputs (efference copy, proprioception) after lesion (Sadeghi et al., 2007, 2010, 2011, 2012). During sensory-motor adaptation, vestibular neurons update their response to altered sensory feedback (Brooks and Cullen, 2014; Brooks et al., 2015; Mackrous et al., 2019). Top right panel: The cerebellum displays synaptic reorganization as early as 24 h following the transition to microgravity, which remains for at least 18 days (Holstein et al., 1999). On earth, following labyrinthectomy, cerebellar neurons lose their ability to discriminate between tilt and translation (Yakusheva et al., 2007).